Great thread....I think my best work is this one:

http://www.renderosity.com/mod/gallery/index.php?image_id=1454667&member

Why?  It has taken everything I have learned in the last 4 months, and compiled it all into a signle image, and even during the contstruction of this peice, I picked up a couple of new techniques.  For me, it pushed the the limits of my current technical abilities in all areas....Modeling, Texturing, Lighting...it pushed me personally in Creativity and Imagination...and with over 4,000 metaballs, and a slew of other standard primitives and appx 40 lights....it also taxed the hell outta my computer!  

(now I am going to go look at everyone else's personal favs...because the only way I have been learning, is to study the great works of those that came along wayyyyyyy before me!)